Modelling of 3D microstructure and effective diffusivity of fly ash blended cement paste

Abstract

Accurate prediction of microstructural evolution and ionic diffusivity in fly ash blended cement paste is significant for practical application and durability design of blended cementitious materials. This paper presents an integrated modelling framework for simulating 3D microstructure and effective ionic diffusivity of blended cement paste with various fly ash replacement levels and water-to-binder (w/b) ratios. A voxel-based hydration model using cellular automaton-like evolution rules was developed to simulate 3D microstructural development of fly ash blended cement, based on which the effective diffusivity was simulated using a lattice Boltzmann (LB) model for diffusion considering the contributions of both capillary pores and gel pores in C–S–H gels to ionic diffusion. A series of experiments were conducted to characterise the morphology of Portland cement and fly ash, hydration process and pore structure of fly ash blended cement paste and measure the effective ionic diffusivity. The simulation results agree well with experimental data in terms of hydration heat, calcium hydroxide content, degree of hydration of fly ash, porosity, and effective diffusivity, which suggests that the developed microstructure-based LB model for diffusion can predict the ionic diffusivity of fly ash blended cement paste with high accuracy. The addition of fly ash can help reduce the ionic diffusivity of cement paste particularly after the capillary porosity depercolation occurs due to the more tortuous diffusion paths in the pozzolanic C–S–H gels.